Method drain

output/java_guava/1.4.19/Queues.java:293–324

Drains the queue as BlockingQueue#drainTo(Collection, int), but if the requested numElements elements are not available, it will wait for them up to the specified timeout. @param q the blocking queue to be drained @param buffer where to add the transferred elements @param numElement

(
    BlockingQueue<E> q,
    Collection<? super E> buffer,
    int numElements,
    long timeout,
    TimeUnit unit)

   */

  @Beta
  @CanIgnoreReturnValue
  public static <E> int drain(
    BlockingQueue<E> q,
    Collection<? super E> buffer,
    int numElements,
    long timeout,
    TimeUnit unit)
      throws InterruptedException {
    Preconditions.checkNotNull(buffer);
    /*
     * This code performs one System.nanoTime() more than necessary, and in return, the time to
     * execute Queue#drainTo is not added *on top* of waiting for the timeout (which could make
     * the timeout arbitrarily inaccurate, given a queue that is slow to drain).
     */
    long deadline = System.nanoTime() + unit.toNanos(timeout);
    int added = 0;
    while (added < numElements) {
      // we could rely solely on #poll, but #drainTo might be more efficient when there are multiple
      // elements already available (e.g. LinkedBlockingQueue#drainTo locks only once)
      added += q.drainTo(buffer, numElements - added);
      if (added < numElements) { // not enough elements immediately available; will have to poll
        E e = q.poll(deadline - System.nanoTime(), TimeUnit.NANOSECONDS);
        if (e == null) {
          break; // we already waited enough, and there are no more elements in sight
        }
        buffer.add(e);
        added++;
      }
    }
    return added;
  }

  /**
   * Drains the queue as {@linkplain #drain(BlockingQueue, Collection, int, long, TimeUnit)},